Prions are comprised of PrPSc, a misfolded isoform of the non-infectious host encoded protein, PrPC. Prion diseases are emerging zoonotic infectious diseases of mammals including humans. Following interspecies transmission, prions can adapt to the new host species resulting in a prion with increased fitness and pathogenicity for the new host species. This process can involve interference between prion strains that are originally present in the inoculum or are generated during interspecies transmission. Recent studies indicate that prions exist as a quasispecies, that is, a mixture of strains. These new studies suggest that prion strain interference may play a larger role in overall dynamics of prion strains. Despite the significance of strain interference in the biology of prion disease, the mechanism(s) underling prion strain interference is not known. We have developed a unique model of strain interference in animals and have recently discovered that in vitro, protein misfolding cyclic amplification recapitulates prion interference. Using these combined approaches, we have determined for interference to occur, the blocking strain PrPSc must be in excess compared to the superinfecting PrPSc in a common cell type and the blocking strain does not convert all of the available PrPC to PrPSc when it prevents superinfecting strain replication. We hypothesize that the blocking strain PrPSc prevents the superinfecting strain PrPSc from gaining access to PrPC or other factors required for prion conversion, therefore inhibiting replication. The aims of this application will investigate if strain interference is due to the blocking strain PrPSc sequestering PrPC from the superinfecting strain or if the blocking strain PrPSc could bind to and prevent the superinfecting strain PrPSc from replicating. The cumulative results of these studies will provide a detailed mechanism of strain interference and have implications for the biology of prion strains and the factors that contribute to maintenance of the prion quasispecies. Finally, the basic mechanisms of how different conformations of PrPSc interact with each other and the cellular environment may be applicable to other protein misfolding diseases of humans.